1. Field of the Invention
The present invention generally relates to laser alignment and, more particularly, is concerned with a new and improved method and apparatus for precisely adjusting the optical elements of a waveguide laser having at least one waveguide segment. Features of the present invention can also be used in non-waveguide laser systems such as free space mode lasers.
2. Description of the Prior Art
Because light can be propagated over relatively large distances in waveguides without an appreciable reduction in power density, waveguide lasers are in increasing demand for medical, industrial, and military applications. In contrast to conventional lasers where feedback and resonator modes are established by normal free space propagation, waveguide lasers incorporate a resonator wherein radiation is transmitted in part by guided wave propagation.
Advantages of waveguide laser systems as compared to conventional lasers include: reduced laser size due to a reduction in bore diameter; increasing gain per unit length with decreasing bore diameter; higher gas pressures resulting in increased optical bandwidth; high pressure operation resulting in potential increased frequency tunability in molecular lasers such as CO.sub.2 ; efficient matching between the laser mode volume and the laser excitation region; and excellent mode control through the unique properties of waveguide laser resonators.
Examples of waveguide gas lasers are disclosed in the following U.S. patents: U.S. Pat. No. 4,103,255 to Schlossberg; U.S. Pat. No. 4,19,251 to Laakmann; U.S. Pat. No. 4,577,323 to Newman et al; and U.S. Pat. No. 4,438,514 to Chenausky et al.
Because increasing the active gain length of a waveguide laser can yield higher output power, folding the waveguide bore of a waveguide laser can increase output power. As such, folded waveguide lasers are increasingly used for compact, high power laser packaging. Folded waveguide lasers are discussed in "Comparison of Waveguide Folding Geometries in a CO.sub.2 Z-Fold Laser," by P. E. Jackson, Applied Optics, 1 Mar. 1989, Vol. 28, No. 5, pp. 935-41.
Folded waveguide lasers typically include an optical cavity (also known as a waveguide bore) extending between two end reflectors. At least one intermediate fold reflector is arranged with the two end reflectors such that a continuous optical path between the two end reflectors is provided by way of successive reflections from each folding reflector in a predetermined sequence; each part of the optical path between consecutive reflecting elements along the length of the optical path defines a waveguide segment. Waveguide lasers also typically include an active medium filling at least some of the waveguide segments in addition to an excitation means for producing laser action in the active medium.
Although folded waveguide lasers can yield longer overall gain lengths in a compact laser package, there are important problems to be solved in the fabrication and alignment of fully optimized folded waveguide lasers. In particular, misalignment of waveguide laser optical elements may result in degraded laser performance as evidenced by reduced laser power, unstable mode changes, reduced beam quality, and beam pointing instabilities. Furthermore, stable, single mode operation of a folded waveguide laser is unlikely to persist if one or more fold reflectors is misaligned. It is therefore necessary to precisely adjust waveguide laser optical elements so that radiation is optimally coupled from one waveguide segment to the other, in order to obtain optimal laser operation. The desired effect is to provide a folded waveguide path which, to the propagating laser radiation, is equivalent to that of a precisely machined waveguide bore produced in a single, straight section which has the full length of the laser cavity.
No satisfactory system has been set forth for precisely adjusting the optical elements of a waveguide laser so that the laser, when operating, is optimally aligned. A known method for adjusting optical elements involves roughly aligning end and fold reflectors during waveguide fabrication; the laser is then filled with gas and the optical elements are aligned through trial and error. This procedure may be acceptable for non-folded waveguide laser system because the degrees of freedom are sufficiently small--a reasonable amount of iterative adjustment may result in optimum laser performance, as evidenced by laser output power, beam quality, etc. This fabrication procedure does not, however, work well for folded waveguide lasers. Because of the added degrees of freedom in a system with two or more waveguide segments (such as a three segment Z-fold waveguide laser having four mirrors, each with two degrees of angular freedom), misalignment of one optical component can be compensated for by misalignment of one or more of the other components, with no way of determining the true status. Such misalignment may manifest itself as hypersensitivity of the laser operation (e.g., switching to undesired modes), in response to temperature or vibration environment. Attempts to improve performance by trial and error are extremely difficult and tedious, even for those highly experienced individuals trained in the fabrication of waveguide lasers.
Another technique for producing a properly aligned waveguide laser is to machine mounts and mounting flanges to sufficient precision so that, during assembly, the optical components will be adequately aligned. This approach has proven unsatisfactory because the required precision is extremely high, making this approach difficult, time consuming, and expensive.
Under current methods, then, the precision alignment of waveguide lasers is extremely difficult to achieve. Procedures heretofore employed for adjusting optical elements depend on trial and error adjustments to a completely fabricated laser.
The present invention employs the laws of diffraction for optimally aligning a waveguide laser. As disclosed in "Physical Optics," by R. W. Wood, published 1988 by the Optical Society of America, pp. 220-222, when parallel light is partially blocked by a thin edge, a diffraction pattern is formed. This diffraction pattern has a bright fringe, brighter than the non-diffracted light, located at the edge of the shadow. Away from the shadow, the intensity approaches that of the light source because no diffraction occurs.
The inventor has observed that diffraction along a waveguide wall is similar to diffraction at a thin edge except that diffraction occurs at many points along a waveguide wall and adds to the maximum intensity of the diffraction pattern. If parallel light propagates parallel to the waveguide and grazes two opposing walls of the waveguide, then the number of diffractions on both walls is equal causing a symmetrical diffraction pattern. If, on the other hand, the light does not propagate parallel to the waveguide walls, then the number of diffractions is greater on one wall than the other, producing a diffraction pattern having a higher peak intensity on one side than the other.
The inventor has further observed that the symmetry of the diffraction is extremely sensitive to the angular alignment between the light and the waveguide walls. For example, in the preferred embodiment of the present invention, deliberate misalignment of parallel light by 0.1 mrad with respect to waveguide walls results in a detectable, unsymmetrical diffraction pattern. Furthermore, tests of the repeatability of setting the alignment of a 450 mm long waveguide show it can be reliably returned to the same position within .+-.0.05 mrad.
Employing the laws of diffraction, the present invention overcomes the limitations of previous alignment techniques by providing a system for precisely adjusting the optical elements of a waveguide laser during fabrication.